Efficient performance of drilling operations makes it necessary to determine an actual path of the well bore drilling without pulling the turbodrill and the drilling string to the surface. For that purpose, use is made of a diamagnetic pipe and instruments which are placed in a special diamagnetic container which is housed in the diamagnetic pipe.
The instruments determine the angle of inclination of the container with respect to a vertical line and the direction of this inclination with respect to the magnetic pole of the Earth (azimuth). The diamagnetic pipe is used as a separator of magnetic masses so as to eliminate their influence on the magnetic part of the instrument determining the azimuth.
The three-dimensional position of the well bore is determined in the following manner.
The diamagnetic pipe is installed in the drilling string over or under the turbodrill as close as possible to the drill bit and is aligned with the well axis. The container with the instruments is lowered into the drilling string and is installed in the diamagnetic pipe, the container being centered with respect to the axis of the diamagnetic pipe.
As a result of the relative centering of the container with instruments in the diamagnetic pipe and the diamagnetic pipe in the well bore, the actual path of the well bore (angle of inclination and azimuth) is determined by the instrument readings.
In order that the actual path of the well bore deviate by the minimum possible amount from the pre-set path, the readings of the instrument determining the actual path of the well bore should be obtained at points which are as close as possible to the rock breaking tool (drill bit). These readings are used for the prompt control of the turbodrill operation and drill bit movement without permitting the divergence between the actual and theoretical paths of the well bore to exceed an allowable value.
Two basic requirements to the turbodrill design may be formulated on the basis of the above considerations.
First, the turbodrill should have a diamagnetic pipe which is used to accommodate measuring instruments for determining the path of the well bore drilling, the diamagnetic pipe having to be rigidly coupled to the drilling string and aligned with the well bore.
Second, the design of the turbodrill should provide for a rational accommodation of the diamagnetic pipe used for housing measuring instruments so that the diamagnetic pipe be at the shortest distance from the rock breaking tool such as a drill bit.
One of the turbodrills used at present which is based on a successive system of drilling fluid flow comprises turbine sections each having a casing and a solid shaft. The turbodrill also comprises a spindle installed in the casing and carrying a rock breaking tool such as a drill bit. The turbodrill has a diamagnetic pipe which is designed to accommodate instruments for measuring the three-dimensional position of the well bore, which is aligned with the path of the well bore and arranged over the turbine sections (cf. M. T. Gusman et al., Calculation, Design and Operation of Turbodrills (in Russian), M., Nedra Publ. House, 1976, p.35, FIG. 9). The provision of the solid shaft and the diamagnetic pipe arranged over the turbine sections in such a turbodrill makes it possible to measure the actual path of the well bore at a distance of 30 to 40 m from the drill bit. This distance from the pipe to the drill bit cannot ensure the prompt control of the bit operation in spite of the fact that the diamagnetic pipe is aligned with the well bore path, since the measurement results are obtained at a distance of 30 to 40 m from the drill bit. The use of such turbodrills becomes inexpedient.
Another turbodrill based on the parallel system of drilling mud flow (cf. USSR Inventor's Certificate No. 121102, 08.12.58) comprises turbine sections each having a casing and a hollow shaft. The turbodrill also comprises a spindle installed in the casing and carrying a rock breaking tool (drill bit). The turbodrill has a diamagnetic pipe which is designed for accommodation of instruments for measuring the three-dimensional position of the well bore, and which is installed under the spindle to transmit axial load and rotary motion to the drill bit. Owing to the fact the turbine section shafts and spindle are hollow, the pipe with the instruments may be installed closer to the drill bit so that measurements of the actual path of the well shaft may be taken at a point closest possible to the drill bit and a prompt control of the turbodrill operation and movement of the drill bit is ensured.
The disadvantage in operation of this turbodrill resides in that, while measurements of the actual path are taken at points as close as possible to the drill bit, they are not accurate enough since the diamagnetic pipe with the instruments is not rigidly coupled to the drilling string and is not aligned with the well shaft because of its use for transmitting rotary motion and axial load to the drill bit.
It does not appear possible to align the diamagnetic pipes accommodating the instruments with the well bore with such an arrangement of the turbodrill, since rotary motion is transmitted to the drill bit at a speed from 2 to 10 revolutions per second which causes a rapid failure of centering members provided on the diamagnetic pipe and spindle, which in turn results in intensive wear of the diamagnetic pipe.
Failure to align the diamagnetic pipe carrying instruments with the well bore results in that the measurements of the three-dimensional position of the bit and turbodrill are not always accurate, and though such measurements are taken at a very close distance from the well bottom the control of the turbodrill operation and movement of the drill bit in space is incorrect. Besides, with this turbodrill it is impossible to drill directional holes, because deflecting devices mounted on the spindle are too far from the bit.